This chapter is most relevant to Section L1(iii) from the 2017 CICM Primary Syllabus, which expects the exam candidates to "describe the mechanism of excitation-contraction coupling". This has appeared at least a couple of times, once for each muscle tissue type (Question 8 from the first paper of 2014 for skeletal muscle, and
Question 18 from the first paper of 2012 for smooth muscle). Both listed SAQs had dismal pass rates (34% and 20% respectively), suggesting the trainees were caught off-guard. Thus, a short answer-like summary is presented here, for future candidates to store in their synaptic granules.
- Excitation-contraction coupling is the series of events that link the sarcolemma action potential to muscle contraction and relaxation
- Action potentials propagate along skeletal myocytes at 3-5m/s
- Skeletal muscle have the shortest APs (4ms), cardiac myocyte APs are longer (200-300 ms), and smooth muscle may have action potentials lasting minutes
- In striated muscle the APs access the centre of the myocyte through T tubules
- Voltage-gated L-type calcium channels are activated by the action potential
- For skeletal muscle, these are connected directly to the ryanodine receptor
- For cardiac myocytes andf smooth muscle, they activate the ryanodine receptor using calcium as a second messenger (CICR, calcium-induced calcium release)
- The ryanodine receptor then acts as a calcium channel, releasing stored calcium from the sarcoplasmic reticulum.
- Calcium release from the sarcoplasmic reticulum occurs
- Cytoplasmic free calcium concentration increases to 20 μmol/L
- Intracellular calcium acts on regulatory proteins
- Troponin C in striated muscle, which dissociates from the actin/tropomyosin complex and permits crossbridge cycling
- Calmodulin in smooth muscle, which activates myosin light chain kinase which then phosphorylates the light chain of myosin, greatly increasing its ATPase activity
- Crossbridge cycling occurs
- Myosin binds ATP, dissociates from actin, and "cocks" its head to a 90º angle.
- Then its head binds actin again, which is the cross-bridge
- It then releases the inorganic phosphate and returns its head to its original position, which results in the movement of the myosin molecule about 11 nm along the actin filament.
- Calcium buffering by proteins removes some calcium from the cytosol
- Free calcium concentration decreases when it binds to troponin, ATP and parvalbumin
- Calcium removal from the cytosol is required for striated muscle relaxation
- Calcium is removed mainly by the SERCA ATPase pump which removes it from the cytosol and returns it to the sarcoplasmic reticulum.
- In the absence of calcium, striated muscle troponin and tropomyosin block the myosin binding sites on the actin filament, preventing cross-bridge formation
- Smooth muscle relaxation
- Smooth muscle myosin continues to function as an ATPase until it is dephosphorylated by myosin light chain phosphatase, which is activated by nitric oxide and cGMP.
To simply throw the string "excitation-contraction coupling" into a market-dominant search engine should usually yield a vast plethora of peer-reviewed and homecooked literature on this subject. The real skill of the reviewer is revealed in the sort of recommendations they can make after filtering through these. A candidate with limited time resources will obviously go for the single shortest simplest explanation, such as this review by Sweeney & Hammers (2018). Another excellent resource is this thing from columbia.edu: it appears to be a scanned copy of a photocopied textbook chapter, distributed as a part of some course in 2004 and exposed for the world to see by some wonderfully porous folder permission settings, presumably dating back to Web 1.0. In-depth professional works encompassing the entire topic are not available, as nobody seems to have the stamina to cover all three tissue types in detail, and the reader needs three separate papers. Calderón et al (2014) is great for skeletal muscle, Stern (1992) for cardiac and Somlyo (1985) or Endo (1983) for smooth muscle (yes, these references are forty years old, but they are free and the for the casual reader the main points remain accurate).
Where do the boundaries of this topic lay? Most unready people, confronted with the question, would still be able to blurt out a passable definition, because the wording of the term is fairly unambiguous. It was coined in 1952 by Alexander Sandow, who used it to designate "the entire sequence of reactions - excitation, inward acting link, and activation of contraction". The nomenclature was clearly successful, insofar as the success of any terminology can be measured by its longevity. Seventy years on, we still have people referring to it by the same words, and as Dirksen et al (2022) have pointed out, the use of the word "coupling" has metastasised into other areas of medicine and biology, such that we see numerous "like-named franchises for secretion, transcription, and other couplings".
The modern reader, looking for a straight answer, would probably not find one better than the definition offered by Smith et al (in UpToDate):
"the series of events that link the action potential (excitation) of the muscle cell membrane (the sarcolemma) to muscular contraction. "
This means the most important points to cover in a written exam answer are:
Additionally, predicting a relentless march towards increasingly more esoteric exam papers, the candidates should prepare to answer questions that ask them to compare the excitation-contraction coupling between the different types of muscle, and this is attempted at the very end.
We could pick up this thread from the generation of the endplate potential by the opening of the nicotinic receptors at the neuromuscular junction, which would be a classical point of reference to start discussing the action potential of muscle cells, but this would be unfairly biased in favour of skeletal muscle. It is probably worth noting that the other muscle tissues have substantially different excitable behaviour. The best reference for this, unfortunately, was a group of chapters from the excellent Cell Physiology Source Book by Nicholas Sperelakis (1995), spanning about 100 pages; and no sane person would ever recommend these as essential exam preparation material. To spare the reader, the main differences can be summarised in a table:
|Skeletal muscle||Cardiac muscle||Smooth muscle|
|Electrical union||Each cell is electrically isolated||Connected by intercalated discs ("electrical syncytium")||Variably connected by gap junctions, depending on the tissue|
|Neural connection||Each cell has its own motor endplate||Most cells are not directly innervated||Most cells are not directly innervated|
|Resting membrane potential||-80 mV||-90 mV||From -50 to -70 mV|
|AP duration||4 ms||200-300 ms||Seconds, minutes...|
|Conduction velocity||2-5 m/s||0.4-0.9 m/s||15-1.6 cm/s|
Skeletal muscle action potentials are ridiculously quick. The typical skeletal myocyte action potential "spike" lasts about four milliseconds, which is only slightly slower than the slowest of neurons. Here's a representative recording of a frog semitendinosus fibre from Sperelakis et al (1973), demonstrating all the important features (as well as some recording artifact):
Cardiac myocytes have a much longer action potential, lasting 200-300 milliseconds. There is already enough written about it in the cardiovascular physiology section, but here's a diagram anyway, in the unlikely case that the reader is somehow unable to recall this very characteristic shape:
This is about a hundred times slower than skeletal muscle, but still relatively quick compared to smooth muscle cells, which can remain depolarised for whole minutes. For example, here is a recording of the transmembrane potential from some vascular smooth muscle being tortured with noradrenaline by Hermsmeyer (1982):
As you can see, this vascular smooth muscle remained depolarised for the duration of the stimulus, and this weird behaviour is typical for vascular smooth muscle cells. In fact there is a lot of weirdness in smooth muscle electrical behaviour:
After all this it will not surprise the reader to learn that the rate at which the action potential propagates along a myocyte will also differ depending on which types of muscle cells are involved. Intact skeletal muscle fibres enjoy rapid conduction, with a minimum action potential velocity similar to that of unmyelinated nerves - something like 2-5 metres per second, according to amphibian muscle experiments by Sheikh et al (2001) and human data by Troni et al (1983). Just as with nerves, the width of a fibre seems to matter - i.e. thicker fibres conduct faster. Cardiac myocytes conduct more slowly, at a rate of perhaps 0.4 to 0.9 m/s, which is fine because the heart is a rather compact organ (compared to, for example, the 60cm long sartorius) and the action potential does not have far to travel. Predictably, the slowest of all are smooth muscle sheets, with conduction velocities ranging from the relatively brisk 15 cm/sec for the human oesophagus to the embarrassingly sluggish 1.6 cm/sec for the retractor muscle of a dog's penis (Burnstock & Prosser, 1960).
For the thick meaty fibres of skeletal and cardiac muscle to contract rapidly in a coordinated fashion, the action potential needs to propagate deep into the interior of the cell, and it does so by means of the T tubules. Hill (1949) famously detected that it takes less than 40 milliseconds for excitation to make its way into the centre of the cell (in his case, a cardiac myocyte with a radius of 50 µm), and this phenomenon remained unexplained until the T tubules were discovered during the explosion of electron microscopy in the mid-1950s. As their importance is tightly linked to the importance of calcium, and the importance of calcium is the topic of the next section, not much more needs to be said about these structures, other perhaps than the statement that they tend to be more abundant where calcium cycling is more rapid. The mouse, whose resting heart rate is 500 beats per minute, has a higher density of T tubules in its cardiac myocytes than slower-paced animals, perhaps to facilitate a faster ion exchange by harnessing a larger surface area (Heinzel et al, 2002). By extension of the same principle, smooth muscle has no need for T tubules, as the smooth muscle cells are relatively small and nothing involving smooth muscle needs to happen especially quickly.
One way or another, intracellular calcium is central to muscle function because it is a necessary cofactor for the contractile apparatus, and it can be made available to those proteins in two ways: it can be acquired from the extracellular fluid, or it can be released from the sarcoplasmic reticulum. Most muscle tissue types tend to do a bit of both. Cardiac myocytes tend to rely on a supply of extracellular calcium for their function, whereas skeletal and smooth muscle tend to rely more on the intracellular calcium already stored in their sarcoplasmic reticulum. As usual, there is no single paper that compares these mechanisms, but a good reference for each muscle tissue type is supplied below.
If you had to abbreviate the whole topic of excitation-contraction coupling to the description of just one mechanism, this would probably be it. These different methods of getting calcium into the myocyte are the main step that connects excitation to contraction, and if time were short, the exam candidate would probably be able to safely compress their answer around this central point and still score a few marks.
Calcium is the final common pathway for all three muscle tissue types. It acts as a signalling molecule by interacting with regulatory proteins that modulate the activity of contractile proteins. Specifically, these regulatory proteins inhibit contraction, unless calcium comes along and turns them off. This mechanism is slightly different in skeletal cardiac and smooth muscle, and the clearest explanation of the differences actually comes from this old textbook chapter, around pages 237-238.
Like in all cells, the calcium concentration in myocytes is closely regulated and the normal concentration of calcium in the cytosol is less than 200 nmol/L (Barry & Bridge, 1993). This is expected behaviour for an intracellular second messenger. Its concentration in the extracellular fluid (around 1.10-1.20 mmol/L) and in the sarcoplasmic reticulum (0.4 mmol/L) drives the gradient for its entry into the myocyte cytosol when the appropriate cation channels open. Though various authors (this one included) might occasionally refer to the resulting calcium influx as some kind of massive inundation, the reality is very different, as the cytoplasmic free calcium concentration might only increase up to something like 20 μmol/L, i.e. 0.02 mmol/L (Baylor & Hollingworth, 2003). This tiny fractional change is all that is required to produce muscle contraction.
So: as mentioned above, with the regulatory brakes removed by calcium, contractile proteins are free to use as much ATP as they like in the process of contraction. That process is so well-described by such a vast range of excellent resources that there is probably nothing left to do but post an animated gif:
This "walk along" process can be explained with words, or images, or video, but (at least in the author's own personal experience) it does not seem to matter how many times, or in how many ways, this process is explained - it will inevitably be forgotten by the vast majority of people shortly after whatever exam they are doing. Ergo, the most important consideration in presenting this information is to make it into easily crammable point-form statements for short term storage and easy regurgitation:
And it absolutely would bind another ATP molecule if it could, because the affinity of the myosin head for ATP is very high. In fact at physiologically normal levels of intracellular ATP the myosin head is never short of ATP, and would just keep repeating this process forever. For this reason we need regulatory molecules to prevent endless contraction. For striated muscle this occurs at the point of cross-bridge cycling, which is prevented by tropomyosin, and in smooth muscle the regulation occurs at the level of ATP hydrolysis which cannot occur until the myosin head is turned into an ATPase by calmodulin. All of these regulatory mechanisms require the absence of calcium to function, which means calcium needs to be rapidly removed from the cytosol to allow relaxation. This is achieved by two main systems: calcium buffering and the sequestration of calcium in the sarcoplasmic reticulum.
Calcium buffering is a method of rapidly decreasing the calcium concentration in the myocyte that requires basically no energy, which is very attractive. As a divalent cation calcium is already attracted to negatively charged regions on protein molecules (just look at how it interacts with albumin in the bloodstream), so to associate with some cytosolic proteins and become biologically inactive would not be a huge change in its normal behaviour. The most important buffers are troponin C itself, ATP, and parvalbumin - a protein that seems to be specifically purposed to buffer calcium, and which acts as a chaperone to deliver it to the sarcoplasmic reticulum. This mechanism plays a variable role in different muscle tissues; it is probably more valuable to fast-twitch skeletal muscle where rapid relaxation is necessary (as these seem to contain more parvalbumin).
Ultimately, binding calcium to cytosolic proteins is not a long-term solution for the intracellular calcium problem. All those ions still need to be disposed of somehow to abolish contraction and to return the muscle to the relaxed state. There are two options: either they get deported by surface membrane pumps, or they get confined in the sarcoplasmic reticulum. The latter is the dominant mechanism, and both options are explored in considerable detail by Berchtold et al (2000).
Transmembrane pumps like the sodium-calcium exchange pump NCX or the ATP-powered calcium transporter PMCA do contribute, but conceptually it would be hard to justify moving all the calcium into the extracellular fluid because it is such an important second messenger, and it would not be beneficial to the organism to have huge exercise-associated fluctuations in calcium levels. It would be better to sequester it into the sarcoplasmic reticulum, and this movement is mediated by the ATP-powered SERCA pumps, where "SERCA" stands for "sarco(endo)plasmic reticulum calcium ATPase". This thing cracks enough ATP molecules to account for something like 40-50% of resting muscle metabolism, or 12-15% of total body oxygen consumption, considering that muscle mass represents perhaps 40% of total body mass.
The function of this pump is regulated by various mechanisms, which differ between muscle tissue types, and only one of which has some vague pharmacological interest for the intensivist. In cardiac muscle, the protein phospholamban inhibits SERCA activity by reducing its affinity for calcium, thus preventing relaxation (MacLennan et al, 2003). However, when demand for cardiac output is increased, the myocardium would need to relax more, not less, to accommodate a larger end-diastolic volume. This is achieved by catecholamines, as they activate protein kinase A which phosphorylates phospholamban and causes it to dissociate form SERCA. The disinhibited SERCA then gobbles up all the calcium and you get a bigger stroke volume to go with your increased contractility. This is the mechanism of catecholamine-induced increase in lusitropy.
Relaxation is therefore an ATP dependent process, as ATP-powered pumps are necessary to remove calcium and allow normal inhibitory activity of cross-bridge cycling regulators. Also the myosin heads need new ATP to dissociate from actin.
In death, where all the myosin heads run out of ATP and there is nobody left to remove calcium from the cytosol, the muscle proteins end up stuck in a rigid contracted state. This is the basis for rigor mortis, which develops early after death, and which persists for perhaps 15-25 hours (until the contractile proteins start to get degraded by lysosomal enzymes).
Merely removing calcium from the cytosol may not stop the process of smooth muscle contraction by itself, as the phosphorylated myosin will just carry on with its ATPase activity even in the absence of calcium. It needs to be dephosphorylated in order for it to stop. This is achieved by myosin light chain phosphatase, an enzyme that is an indirect target for smooth muscle relaxants that act via the nitric oxide and cGMP pathways. Nitric oxide, a soluble extracellular mediator, penetrates into cells and activates guanylate cyclase, which produces cyclic GMP, which then activates protein kinase G, which in turn phosphorylates myosin light chain phosphatase, activating it and bringing about smooth muscle relaxation.